manageddisplay: fixed segfault when closing exportdialog

Really affected any color managed display widget, including scopes, viewers,
and the export dialog.

app/widget/manageddisplay/manageddisplay.cpp

@@ -32,6 +32,11 @@ ManagedDisplayWidget::ManagedDisplayWidget(QWidget *parent) :
   setContextMenuPolicy(Qt::CustomContextMenu);
 }
 
+ManagedDisplayWidget::~ManagedDisplayWidget()
+{
+  ContextCleanup();
+}
+
 void ManagedDisplayWidget::ConnectColorManager(ColorManager *color_manager)
 {
   if (color_manager_ == color_manager) {

app/widget/manageddisplay/manageddisplay.h

@@ -35,6 +35,8 @@ class ManagedDisplayWidget : public QOpenGLWidget
 public:
   ManagedDisplayWidget(QWidget* parent = nullptr);
 
+  virtual ~ManagedDisplayWidget() override;
+
   /**
    * @brief Disconnect a ColorManager (equivalent to ConnectColorManager(nullptr))
    */